What Causes Delayed Periods While Using Nuva Ring?

Hi, I'm currently on the Nuva Ring and have been for almost 9 months. Last month I did not have a period at all. My period was supposed to have started Sunday the 18th but it started Tuesday the 13th and hasn't let up. I'm aware it could be my birth control but this hasnt happened since I've been on it. What could be the reason?